古生物学：「シカゴ」始祖鳥が、この古代鳥に新たな知見をもたらす

A suite of previously unknown features belonging to the oldest known fossil bird, Archaeopteryx, is revealed in Nature this week. Analysis of the Chicago Archaeopteryx uncovers new information about the skeleton, soft tissues and plumage of this iconic taxon. The findings shed new light on the bird’s ecology, as well as the evolutionary transition from non-avian dinosaurs to modern birds.

The Chicago specimen, so-called because it was acquired by the Chicago Field Museum of Natural History, is the 14th known specimen of Archaeopteryx. Jingmai O’Connor and colleagues imaged the well-preserved three-dimensional remains, which are almost complete, missing just a single digit. Specialized inner secondary feathers, called tertials, can be seen on both wings. Paired bones called proatlases sit between the skull and vertebral column. The skull is less rigid and the tail is longer than was previously recognized. Soft tissue analysis reveals that the minor digit on the hand was free and able to move, and the shape of the bird’s foot pads indicates that it was adapted for moving on the ground.

Non-avian dinosaurs that are closely related to birds do not have tertial feathers on the upper part of their wing. The authors posit that their presence in the Chicago specimen suggests that these feathers may have evolved for flight, creating a continuous aerodynamic surface. Along with the other features, these findings point towards a bird that was adapted for some level of flight and had a mixed terrestrial and arboreal ecology.